XCode 8 app building ultraslow

Problem is that when building a Swift app in XCode 8, the source files are compiled quickly, but Copying Swift standard libraries takes the cake - the phase could run for several minutes (usually the whole build is around 4-5 minutes, worst cases have been generously over 10 minutes – used to be under a minute).

Additionally, the signing phase of archive's export is painfully slow, sometimes taking close to half an hour (again, used to be quite quick).

I've scoured the web for answers for days, reinstalled XCode countless times, tried new Swift toolchains, experimented with turning on/off Sierra's iCloud Drive (the derived folder still needs to be hidden from it), tried every build setting that anyone, and their mother, has recommended, made numerous new blank projects, and lost all hope...

So my question is, how can i make it normal?

EDIT:
I also have 867Apple ID keys (same amount of pub-s and priv-s, and the accompanying certificates) in my Mac's Keychain – is there a chance this issue is related to it? Takes a ton of time to go through them and find the correct one to sign with?